1. Field of the Invention
The present invention relates to a signal suppressing apparatus suppressing undesired signal components.
Devices for controlling the suppression of undesired signals such as echo cancelers, noise cancelers, ghost cancelers, active noise control systems, active vibration control systems, active suspensions, etc. have now spread in general use. In these systems, when creating a signal processing system, there is a problem in that there is a time lag in the signal processing at that processing system and this obstructs realization of the phase-characteristics of the transfer function needed. For this reason, it is necessary to increase the speed of operation of the processing system and shorten the time lag produced in the signal processing as much as possible.
2. Description of the Related Art
The processing system of the signal suppressing system mentioned above makes frequent use of multiplication and accumulation operations and therefore usually is constituted by a digital signal processor (DSP). In recent years, high speed operation has become possible in DSP's, but tasks are processed in a single task format (processing for sequentially executing routines wherein after a certain single routine in the DSP software is ended, the next routine is started). Unlike with a general purpose CPU, it is not possible to simultaneously execute a plurality of processings in a multi-task format.
As mentioned above, current DSP's all perform processing by the single task format, and therefore the processing speed depends on the capability of the DSP. For this reason, it is necessary to provide a high speed DSP for performing high speed processing. As a result, the cost for realization of the processing system naturally rises. In addition, this means that there are time delay elements in the processing system, which become an obstacle in the realization of the phase characteristics of the intended transfer function and become an obstacle to optimal control.
For example, consider a case where the signal suppressing apparatus is applied to an active noise control system aimed at random noise having non-periodic components. In this active noise control system, when the noise from a noise source is emitted to the outside by propagation through a duct etc., a sound cancelation signal having the same magnitude and inverse phase to the noise is prepared by a digital filter (cancelation filter) comprising a DSP, and this is superposed on the noise by a speaker etc. near the outlet of the noise to cancel the noise, but if there is a large time delay in the signal processing for creating the sound cancelation signal by this cancelation filter, where the noise propagation path is short, the cancelation sound is not created at the same timing as the noise to be canceled, and therefore the noise cannot be canceled. As a result, there arises an inconvenience in that this system can only be applied to systems having a long noise propagation path and a large size.